Lava flow trail stop 5

How to Build a Cinder Cone

What did the eruption of Sunset Crater Volcano look like? This photo of Pu`u `O`o cone erupting in Hawaii might help you imagine it.

Start with the right ingredients

To make a cinder cone, you need to have lava with lots of gas. Gas-rich lava expands as it travels toward the surface. If you shake up a soda, then open it, you can simulate this effect. Open the can, or unplug the volcano, and you release the pressure that holds the gas in and -WHOOSH!- the molten rock sprays into the air as a fiery fountain.

That brings us to the second critical cinder cone ingredient, runny lava. Only very fluid lava with lots of gas can be sprayed hundreds of feet into the air as lava globs to form a nice cone.

Basaltic lava is exactly the right stuff! It has a low silica content, so it is very runny (the amount of silica in molten rock determines how runny or thick the lava will be).

This view shows the of curtains of fire that erupted prior to the growth of Pu`u `O`o cone in Hawaii. The initial eruption at Sunset Crater may have looked very much like this.

Curtains, please!

The basalt lava that erupted to make the Sunset Crater cinder cone made its way to the surface through a deep fracture that tapped a magma-filled pocket (magma chamber) beneath the sedimentary layers you've seen at the Grand Canyon.

Rather than spewing from a single vent, lava gushed onto the scene where this deep fracture cut the surface. A glowing 'curtain of fire' may have extended over several miles before eruptions focused their attention on the vents that built Sunset Crater Volcano.

Fiery fountains

As the outpouring of lava became focused beneath what is now Sunset Crater, lava fountains threw blobs of molten basalt hundreds of feet into the air. Although lava erupted at 1200° centigrade (2200° Fahrenheit), most airborne molten globs cooled and solidified to form cinders before they reached the ground. Most cinders fell very near the central vent, building a small cone.

Cinder by cinder

Layer upon layer of volcanic ejecta are laid down, building a higher and steeper cinder cone.

Gravity attacks!

Eventually the cone gets so steep that the sides collapse under their own weight. The collapsing cinders come to rest when they reach just the right steepness to keep them stable. This angle, usually about 35°, is called the angle of repose. Every pile of loose particles has a unique angle of repose, depending upon the material it's made from. Because cinders everywhere tend to have nearly the same angle of repose, cinder cones everywhere develop very similar cone shapes with nice, straight sides rising at an angle of about 35° from the ground.

The final stage of the cone's development

Cinder cones can grow very large, even wider in diameter than their maximum ejection distance. How can this happen? As cinders continue to pile high near the central crater, they will continually collapse. Eventually the collapsing sides can spread a considerable distance beyond the 'throwing distance' of the cone-but they always maintain the same straight sides at just the right shape.